Without immigration, our population would be aging (on average) and in decline. Instead, we're getting younger and the number of people is going through the roof. Is that desirable? No way! But the socialists love it. And they're winning the argument, fait accompli.
Immigration is not having much of an effect upon the age of our population. The average age of an immigrant is 29. Because of chain migration, many immigrants are aged parents and relatives.
At the current level of net immigration (1.25 million a year), 61 percent of the nation’s population will be of working age (15-66) in 2060, compared to 60 percent if net immigration were reduced to 300,000 a year.
If net immigration was doubled to 2.5 million a year it would raise the working-age share of the population by one additional percentage point, to 62 percent, by 2060. But at that level of immigration, the U.S. population would reach 573 million, double its size in the 2000 Census.
FYI: Our current total fertility rate is 2.09 children per female or just at replacement level. Even if we stopped all immigration, legal and illegal, the US population would increase 62 million by 2060.